JAMES B. WYNGAARDEN, M.D.Director, National Institutes of Health,Bethesda, Maryland

VOLUME IPATHOPHYSlOLOGYThe Biological Principles of Disease

VOLUME IIMEDICAL MICROBIOLOGY AND INFECTIOUS DISEASES

Volumes I and II of the International Textbook
of Medicinehave been conceived and written to follow a logical
pedagogical approachand can readily be used in conjunction with theCECIL TEXTBOOK OF MEDICINE.

Vitamin B12. In 1926,
Minot and Murphy observed that it was possible to treat pernicious anemia
by feeding raw liver. In 1929, Castle discovered that the intestinal absorption
of the anti-pernicious-anemia principle of liver, which he called extrinsic
factor (vitamin B12), depended on its first being bound to a factor secreted
by the gastric mucosa, which he called intrinsic factor. Further developments
included the identification of the chemical structure of vitamin B12 and
the elucidation of its mechanism of action.Vitamin B12 is synthesized
only in certain microorganisms.
Animals depend on microbial synthesis
for their vitamin B12 supply. Human foods that contain [high levels
of] the vitamin are of animal origin (meat, liver, fish, eggs, and
milk). The average daily diet in Western countries contains 5 to 30 ~g
of vitamin B12, of which 1 to 5 ~g is absorbed. The total body stores of
the vitamin in adults range from 2 to 5 mg, of which approximately 1 mg
is found in the liver. The daily dietary requirement is about 2 to 3 p.g.
Hence, after stopping intake of vitamin B12, or after total gastrectomy,
it takes several years to deplete the body stores and for the symptoms
of vitamin B12 deficiency to develop. The structure of vitamin B12,
or cyanocobalamin, was elucidated in 1955 by the x-ray crystallographic
analysis of Dorothy Hodgkin. The vitamin B12 molecule (Fig. 16) consists
of two main parts: (1) a planar group, a ring structure surrounding the
cobalt atom, that resembles the porphyrin ring of heme except for a bond
linking two pyrrole rings directly together instead of through a bridging
carbon atom-this structure is called a corrin ring; and (2) a nucleotide
group containing the base 5,6 dimethylbenzimidazolyl and phos­phorylated
ribose esterified with 1 amino, 2 propranol. Finally, a cyanide group is
carried by the trivalent cobalt atom, which is also linked to the benzimidazole
base. The term vitamin B,2 may be used to describe cyanocobalamin,
although it is often used to include other forms of the vitamin in which
the -ON is replaced by another side group. In nature, vitamin B12 exists
largely as the two forms, methylcobalamin and 5’deoxyadenosylcobalamin.
These are labile [easily changed] and are converted to a fourth form of
vitamin B12, hydroxyco­balamin. Both cyanocobalamin and hydroxycobalamin
are used therapeutically.

*0.5 mg/dl bloodI24 hours.

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Dietary vitamin B12 is released
from protein and peptide complexes in the stomach, where it attaches to
both intrinsic factor and a second vitamin B12 binding protein called R-binder.
Intrinsic factor is a glycopro­tein that contains 15 per cent carbohydrate
and is secreted by the parietal cells of the body and the fundus of the
stomach. One molecule of intrinsic factor binds to one molecule of vitamin
B12 in the region of the edge of the corrin ring. The R-protein is degraded
by pancreatic secretions and the vitamin B12 released binds to further
intrinsic factor. This step is impaired in patients with pancreatic
disease, leading to reduced vitamin B12 absorption. The normal site
of absorption appears to be the distal ileum. It is possible to absorb
up to 1.5 pg of vitamin B12 from a single oral dose of intrinsic factor-vitamin
B12 complex. Following absorption, the ileum is reffactory [resistant]
to further vitamin B12 absorption for several hours. At a neutral pH and
in the presence of talcium ions, intrinsic factor-vitamin B12 complex passively
attaches to receptor sites on the brush borders of the ileal mucosa. It
then enters the mucosal cell, where it is found in the cytosol. After exit
from the ileal enterocyte, vitamin B12 is attached to a second major vitamin
B12 transport protein (trans­cobalamin II, see below) which is synthesized
in the ileal cells. Thereafter, the vitamin is transferred to the portal
blood, the peak blood level being reached only 8 to 12 hours after an oral
dose. Intrinsic factor does not enter the portal blood and its fate is
not absolutely clear. Only about 1 per cent of an oral dose of the order
of 30 to 300 p.g of vitamin B12 is absorbed in the absence of intrinsic
factor. This form of absorption occurs throughout the gut by simple passive
means. Vitamin B12 is transported
on specific binding proteins called transcobalamins (TC), of which three
forms, TOI, II, and III, have been isolated. TOII is the main delivery
protein that transports cobalamins to tissues such as marrow and in the
nervous system. It is synthesized by a variety of cells, including macro­phages,
ileal enterocytes, and the liver. The TOIl­vitamin B12 complex attaches
to a receptor site on the surface of various target cells and is internalized
by pinocytosis. TOII is essential for vitamin uptake by cells, and in its
absence a severe vitamin B12 deficiency state develops. The functions of
TO’s I and III, which

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are probably produced by leukocytes, are unknown.
The TO levels vary in a variety of disease states. As might be expected,
TOI levels rise markedly in various forms of granulocytic leukemia and
other myeloprolif­erative diseases and also during the leukocytosis
of infection. Increased TOII levels are observed in liver disease and pregnancy. The metabolic functions
of vitamin B12 are only understood in three well-defined reactions. One
is the conversion of homocysteine to methionine, as shown in Figure 17.
The enzyme involved is homocysteine­methionine methyl transferase,
and the reaction requires 5-methyl tetrahydrofolate as a methyl donor,
S. adenosylmethionine, and the reducing agent (FADH2) as well as methyl-B12
as a coenzyme. Vitamin B12-dependent methionine synthesis is important
in the regeneration of tetrahydrofolate from methyltetrahy­drofolate.
The second reaction involving vitamin B12, as deoxyadenosylcobalamin, is
the conversion of pro­prionate to succinate, as shown in Figure 18.
This reaction is part of the route by which cholesterol and odd-chain fatty
acids as well as a number of amino acids and thymine are used for energy
requirements via the Krebs cycle. Patients with vitamin B12 deficiency
may excrete excess methylmalonic acid. Finally, vitamin B22 is involved
in the i~omerization of j3-leucine to ct-leucine; in vitamin B12 deficiency
13-leucine accumulates while ct-leucine is decreased. The inter­relationships
between vitamin B12 and folate metabolism in the synthesis of DNA are considered
later-it is this function of vitamin B12 that undoubtedly accounts for
the megaloblastic erythropoiesis and related phenomenon of abnormal DNA
synthesis found in vitamin B12 deficiency states. Vitamin B12 is also
essential for the function of metabolic pathways in the central nervous
system.